Tutorial on Antenna Design—Part 3

When developing antennas, engineers need to understand matching, gain, fading, and radiation patterns.

By: Engineering Staff, Linx Technologies

Note: This is Part 3 of our tutorial on antenna design. In Part 1 (see Tutorial on Antenna Design—Part 1), we focused on transmitter/receiver antennas and transmission lines. In Part 2 (see Tutorial on Antenna Design—Part 2), we covered antenna operation and determining length. Now let's look at matching, gain, polarization, fading, and some other issues.

Contents
•Antenna matching
•Radiation pattern
•Gain
• Polarization
•Fading
•A number of challenges

In addition to broad concepts of antenna function outlined in Parts 1 and 2, there are specific issues of antenna performance that are equally important to consider. The most important of these issues are matching, radiation pattern, gain, polarization, and multipath effects.

Antenna matching
Antenna resonance should not be confused with antenna impedance. The difference between resonance and impedance is most easily understood by considering the value of VSWR at its lowest point. The lowest point of VSWR indicates the antenna is resonant, but the value of that low point is determined by the quality of the match between the antenna and the transmission line it is attached to. This point of attachment is called the feedpoint.

By looking at Figure 4, an engineer will notice both antenna (A) and antenna (B) are resonant; however, antenna (B) exhibits a much lower VSWR because the feedpoint impedance of (B) is more closely matched to the impedance of the transmission line. Thus, an antenna must be both resonant and matched for maximum RF energy to be propagated into free space.

Radiation pattern
The term radiation pattern is used to define the way in which RF energy is distributed or directed into free space (Figure 5). The term isotopic antenna is commonly used to describe an antenna with a theoretically perfect radiation pattern. In other words, an isotopic antenna is a product that radiates EM energy equally well in all directions.

Isotropic antennas are, of course, only theoretical and have never actually been built. But, the isotopic model serves as a conceptual standard against which "real world" antennas can be compared.

The real world antenna will efficiently radiate RF energy in certain directions and poorly in others. The point(s) of greatest efficiency are called peaks while the areas of no field strength are called nulls. The overall distribution characteristics of the antenna make up the radiation pattern.

In many applications it is advantageous to have the antenna perform equally well in all directions. In these instances a designer would choose an antenna style with an omnidirectional radiation pattern. In instances where highly directional antenna characteristics are needed an antenna style such as a yagi would be chosen.

Gain
The term gain refers to the antenna's effective radiated power as compared to the effective radiated power of some reference antenna. When the isotopic model is used, the gain will be stated in dBi (meaning gain in dB over isotopic). In instances where the gain is being compared to a standard dipole, the rating will be stated in dBd (meaning gain over dipole).

The generally accepted variation between an isotopic point source and a standard dipole is 2.2 dB. Thus, an antenna rated as having 15 dBi would indicate that the antenna had 15 dB of gain over an isotopic source or 12.8 dB of gain as compared to a standard single-element dipole.

The term gain is commonly misunderstood. Many engineers construe gain to mean an increase in output power above unity. Of course, this is impossible, as the effective radiated power would be in excess of the power originally introduced into the antenna.

The simplest way to understand gain is to think of a focusable light source. Assume that the light output is constant at all times and focused so the light covers a wide area. If the light were refocused to a spot setting, it would appear substantially brighter because all of the light energy is concentrated into a small area. Even though the overall light output has remained constant, the light will have a gain in lux at the focus point over the original pattern.

In the same way, an antenna that focuses RF energy into a narrow beam can be said to have gain (at the point of focus), over an antenna that radiates equally well in all directions. In other words, the higher an antenna's gain the narrower the antenna's radiation pattern.

Polarization
The effective polarization of an antenna is an important characteristic in antenna design. Polarization refers to the orientation of the lines of flux in an electromagnetic field. When an antenna is oriented horizontally with respect to ground it is said to be horizontally polarized. Likewise, when it is perpendicular to ground it is said to be vertically polarized.

The polarization of an antenna normally parallels the active antenna of an element. Thus, a horizontal antenna radiates and best receives fields having horizontal polarization while a vertical antenna best radiates and receives fields having a vertical polarization. If the transmitter and receiver's antennas are not oriented in the same polarization, a certain amount of effective radiated power cannot be captured by the receiving antenna.

In many applications involving portable devices, there is little control over the antenna orientation. But, to achieve maximum range the antennas should be oriented with like polarization whenever possible.

Fading
Multipath fading is a form of fading caused by signals arriving at the receiving antenna in differing phases. This effect is due to the fact that a signal may travel many different paths before arriving at the antenna (Figure 6).

When multipath fading occurs, some portions of the original signal may travel to the receiver's antenna through a direct free space path. Others that have been reflected travel longer paths before arrival.

The longer path taken by the reflected waves will slightly delay their arrival time from that of the free space wave. This creates an out-of-phase relationship between the two signals. The resulting voltage imposed on the receiving antenna will vary based on the phase relationship of all signals arriving at the antenna. While this effect is environmental and not related directly to the antenna, it is still important to understand the role multipath may play in theoretical versus realized antenna performance.

A number of challenges
Designers face a number of challenges not normally encountered in antenna design when developing today's wireless systems. Since many of today's wireless products are compact and portable, a designer may have to balance the issue of antenna performance with issues such as packaging and cosmetic considerations. In addition, today's wireless systems, such as part 15 devices, also place some unusual restrictions on the actual antenna design.

Thus, during the design process, the antenna should be viewed as a critical component in system performance. By gaining a detailed understanding of how an antenna operates, engineers can efficiently develop and implement these products in their system designs.

About the Author:
Engineering Staff, Linx Technologies, 1089 Medford Center, Bldg. 137, Medford, OR 97504. Phone: 800-736-6677; Fax: 541-471-6251.